Piloting any aircraft requires a knowledge of its speed relative to the air, that is to say, to the relative wind. This speed is determined using probes for measuring the static pressure Ps and the total pressure Pt, and sensors for measuring the angle of incidence α and the angle of slideslip β. α and β provide the direction of the speed vector in a reference system, or coordinate system, linked to the aircraft and Pt−Ps provides the modulus of this speed vector. The four aerodynamic parameters therefore make it possible to determine the speed vector of any aircraft, such as, for example, an aeroplane or a helicopter.
The measurement of the total pressure Pt is usually done when using a tube called the Pitot tube. This is a tube that is open at one of its ends and obstructed at the other. The open end of the tube roughly faces the flow.
The stream of air situated upstream of the tube is progressively slowed down to almost zero speed at the tube inlet. The slowing down of the air speed increases the air pressure. This increased pressure forms the total pressure Pt of the air flow. Inside the Pitot tube, the prevailing air pressure is measured.
To be able to function under wet or icing conditions, this Pitot tube is electrically reheated. The reheating prevents the tube from being obstructed by ice, during flights in icing conditions. This tube is provided with one or more drain holes and water traps, to avoid any risk of water build-up in the total pressure tubing, in case of flight in a water-filled atmosphere (passing through clouds, rainy conditions, etc.). The drain holes make it possible to evacuate any liquid water that penetrates into the Pitot tube.
The nominal operation of the Pitot tube requires reheating in order to avoid any accumulation of ice and to prevent the drain holes from becoming totally or partially blocked, which would be detrimental to their function which is that of discharging water that penetrates in flight or on the ground.
In current aircraft, whether the reheating is functioning properly is checked mainly by monitoring the electric current consumed by each probe. In effect, the reheating is generally done by means of a resistive element embedded in the body of the probe and whose resistivity changes with temperature. A resistive element whose resistance increases with temperature can be chosen. Consequently, if the current consumed by the resistive element is greater than a certain value, then the probe is considered to be working, otherwise the pressure information that it delivers is not taken into account and the probe is considered to be defective. In flight, the reheating means consumes a substantial amount of electrical power, but this is not always necessary.